Articulated plunger rod pump

ABSTRACT

A reciprocating rod pump system, comprising an articulating plunger assembly having a plurality of plunger segments, each having a pin end and a box end, and a ball and socket articulating connector formed between the pin end of one plunger segment and the box end of another plunger segment. The articulating connector is formed from a collar rigidly attached to the pin end, which defines a socket, the collar comprising an internal shoulder which abuts an external shoulder on the plunger pin end, such that the uppermost end of the collar does not contact the plunger segment, and a plunger nipple comprising a ball adapted to fit and rotate within the socket, and a shank which rigidly engages the box end thread of a plunger segment.

FIELD OF THE INVENTION

The present invention relates to reciprocating rod pumps and improved components of a rod pump system.

BACKGROUND

Horizontal well drilling has been a significant development in the evolution of the tools and techniques to recover and lift reservoir products to the surface. However, horizontal well drilling continues to pose significant challenges associated with producing oil and gas to surface. These methods are analogous in many ways to creating a pipeline located several hundred meters below the surface of the earth. While in pipeline flow/production operations, there are booster pumps and compressor stations strategically located along the line in order to aid movement of the product along the line, such flow enhancements are not available downhole. Intervention techniques such as pipeline pigging are also available to assist in surface pipeline productivity. None of these techniques are available or practical for use in the production of horizontal oil wells.

In conventional rod pumping activities, it is well known that rod and tubing wear associated with reciprocating rod pumps can lead to premature failure of a rod pumping system. Excessive tubing wear can also lead to costly well workovers and high operating costs. All this is well known in vertical rod pumping systems and is even more relevant in high angle rod pump applications. The challenges of pumping at high angles and solutions to those challenges are not well understood to date.

To efficiently produce horizontal wellbores, operators continually require deeper landing depths and higher (more horizontal) angles for their rod pump systems. This minimizes the back pressure residing against the formation and results in the well producing in a “pumped-off” state. In addition, the flow complexity of multi-phase flowing conditions in the region of the wellbore transitioning from substantially horizontal to substantially vertical results in complex, turbulent and disorganized flow which dramatically impacts pumping efficiency. Therefore, landing pumps as deep as practical and preferably horizontally in the wellbore is advantageous. Drilling activities of these wells to accommodate pump landing depths may incorporate a tangent section where the build rates of the horizontal remains constant and therefore creates a linear but deviated section of wellbore in which the rod pump is landed and can reciprocate. As reservoir pressure declines, the rod pump landing depth is moved downward, chasing the declining fluid level. This often results in pumps being landed beyond the end of the tangent section in the wellbore, in a section where there may be large build rates. Operation of a conventional rod pump at high angle build rates beyond the tangent segment of the wellbore can result in substantial wear to the components of the rod pumping system. Numerous components can suffer the effects of this type of application including, but not limited to the pump barrel, plunger, valve rod, or plunger seals.

When landed in a high angle section, the stiffness of the plunger in comparison to that of the barrel can lead to the barrel deflection being substantial enough that the plunger will not easily pass through the barrel, causing exaggerated rod loads, particularly on the upward stroke of the pump. This exaggerated loading condition will lead to a high stress due to bending residing in the valve rod at the terminal top end of the rod pump. This condition can result in the plunger becoming stuck in the pump barrel and the valve rod ultimately failing and parting near the rod pump top or within the pump barrel.

Therefore, there is a need in the art for modified rod pumps which mitigates some or all of the difficulties associated with the prior art.

SUMMARY OF THE INVENTION

The present invention relates to a downhole rod pump system and more particularly improvements to a rod pump system for more efficient operation in high angle or horizontal conditions, such as below a conventional tangent section leading into the substantially horizontal portion of said wellbore. The system may be deployed in conjunction with conventional, unconventional or enhanced oil recovery techniques such as steam-assisted gravity drainage, miscible flood, or steam (continuous or cyclic), gas or water injection.

In one aspect, the invention may comprise rod pump system comprising an articulating plunger assembly for use in a reciprocating rod pump, the articulating plunger comprising:

(a) at least two plunger segments, each comprising a pin end and a box end, wherein the pin end has an external threaded portion and the box end has an internally threaded portion;

(b) a ball and socket articulating connector, formed between the pin end of one plunger segment and the box end of another plunger segment, which articulating connector comprises:

-   -   i. a collar rigidly attached to the pin end, the collar and pin         end together defining a socket, the collar comprising an         internal shoulder which abuts an external shoulder on the pin         end, such that the uppermost end of the collar does not contact         the plunger segment;     -   ii. a plunger nipple comprising a ball and a shank, wherein the         ball is adapted to fit and rotate within the socket, and the         shank comprises an external thread which rigidly engages the box         end thread of a plunger segment, wherein the ball comprises a         travel limiting abutment which contacts a lower surface of the         pin end at the rotational travel limit of the ball within the         socket, and the shank and collar are configured so as to not         contact each other at the travel limit.

In another aspect, the invention may comprise a rod pump system comprising a top rod articulating joint for connecting between a reciprocating rod string and a valve rod comprising:

(a) a rod sub configured to connect to the rod string and having a lower bearing surface;

(b) a swivel sub having an upper bearing surface and lower internal threaded portion and a collar (110) which engages the rod sub to retain the swivel sub, which can freely rotate within the collar (110);

(c) a rod bushing having a lower end adapted to connect to a valve rod and a threaded upper end which rigidly engages the swivel sub internal threaded portion; and

(d) at least one pin, disposed longitudinally within the swivel sub and the upper end of the rod bushing.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the specification and are included to further demonstrate certain embodiments or various aspects of the invention. In some instances, embodiments of the invention can be best understood by referring to the accompanying drawings in combination with the detailed description presented herein. The description and accompanying drawings may highlight a certain specific example, or a certain aspect of the invention. However, one skilled in the art will understand that portions of the example or aspect may be used in combination with other examples or aspects of the invention.

FIG. 1 shows a cross-sectional view of one embodiment of a rod pump assembly of the present invention.

FIG. 2 displays a travelling assembly having 3 plunger segments deflecting in a working state.

FIG. 3 is a partial section view showing a ball and socket articulating connector in a deflected position.

FIG. 4A is an enlarged depiction of the ball and socket joint of FIG. 3. FIG. 4B is a sectional view of a collar in isolation. FIG. 4C is a view of a plunger nipple in isolation.

FIG. 5 is a detailed view of box D in FIG. 1, showing a transverse section view of the top section of an articulated assembly within the rod pump system

FIG. 6 is a detailed view of box C in FIG. 1, showing one embodiment of a top articulating joint assembly.

FIG. 7 shows an exploded view of the top articulating joint assembly of FIG. 6.

FIG. 8 displays a transverse cross section of a swivel sub hex head in the top articulating joint assembly of FIG. 6.

DETAILED DESCRIPTION

The present invention relates to rod pump systems generally, and in some embodiments, more particularly to a system comprising an articulated plunger, which may be used in circumstances resulting in high angle conditions, which may result in relatively large deflections of the pump barrel. The articulated plunger has at least one, and preferably multiple deflection points along its length.

The guidelines and standard practices governing the art of rod pumping within the energy market are well known, and are typically governed by the American Petroleum Institute (API) standards. A component of such guidelines is a standardization of the threaded connections which make up the various fittings and standard components of a typical rod pump system. These standards and guidelines provide consistency and standardization, however they can present significant difficulty when innovation is desired. It is an object of some embodiments of the present invention to conform to standardized threaded connections. The claimed invention described herein comprises embodiments of a rod pump system which may be configured to suit many different wellbore configurations and fluid compositions, and the components of the system. As described herein, the term “longitudinal” or “axial” refers to a direction, axis or plane parallel to the longitudinal center axis of the pump assembly. The term “transverse” or “radial” refers to a direction, axis or plane which is perpendicular to the longitudinal direction, axis or plane. The terms “up”, “above”, “down” or “below” (or the like) are used for convenience to refer to the relative configuration of components when the pump assembly is viewed in a vertical configuration, but is not intended to limit the use of the pump assembly vertically, horizontally, or at any deviated angle.

In one aspect, the invention may comprise a rod pump system as shown in FIG. 1. In one embodiment, an upper portion of the pump assembly includes a top articulated joint (10) connected between the rod string (1) and the valve rod (12). The valve rod (12) extends downward through the rod pump top guide (14) and an upper housing (16). The upper portion of the rod pump may include a sliding valve assembly (2), as described in Applicants co-owned U.S. Pat. No. 10,329,886, entitled “Rod Pump System”, the entire contents of which are incorporated herein by reference, where permitted.

A seating cup assembly (18) is attached to the upper housing (16) and includes seating cups which land the rod pump into a pump seating nipple (112) (not shown) as is well-known in the art. The pump barrel (20) extends downwards from the seating cup assembly and terminates at the lower end of the rod pump with a standing valve assembly (22).

In some embodiments, the upper portion of the rod pump may also include a bushed barrel assembly for centralizing and guiding the valve rod (12) as it enters and exits the pump barrel (20). Such an assembly is also described in Applicants co-owned U.S. Pat. No. 10,329,886, entitled “Rod Pump System”.

The valve rod (12) attaches to a plunger (100) which reciprocates within the pump barrel (20). The plunger is comprised of a plurality of plunger segments (102), connected by articulating connectors (108), as described further below. The upper end of the plunger (100) attaches to the lower end of the valve rod (12) by a threaded connection to a top plunger adapter (101) this item number is missing in the drawings. The uppermost plunger segment comprises a box end female thread, while the top plunger adapter comprises a male pin threaded end. The top plunger adapter then connects to the valve rod by means of a top plunger cage (202).

The bottommost plunger (100) segment is attached to a traveling valve assembly (24). The standing valve and traveling valve assemblies (22, 24) may comprise any suitable standing or traveling valve assemblies. In some embodiments, the standing and traveling valves are configured as described in Applicants co-owned U.S. Pat. No. 10,329,886, entitled “Rod Pump System”.

As is well known in the art, the plunger (100) is bored through end-to-end to allow flow of production fluids through from the pump barrel (20) below the travelling valve (24) to the plunger bore (3) above the travelling valve.

The plunger (100) is articulated, meaning that it is capable of bending slightly along its length to accommodate deflection in the pump barrel (20). In one embodiment, the articulated plunger (100) comprises at least two plunger segments (102), joined in a manner which permits some deflection between two adjacent segments, and which tolerates the forces transmitted through the plunger (100) during pumping operations. As used herein, “deflection” means that the longitudinal, central axes of adjacent plunger segments are not co-axial and parallel, but rather a small angle is created between the two axes. Accordingly, an articulating connector (108) is disposed between and attached to the at least two plunger segments, and permits at least one deflection point relative to one or both of the at least two plunger segments as may be seen in FIG. 2. In some embodiments, this articulating connector (108) may provide up to about 5° of deflection between adjacent plunger segments.

In some embodiments, at least one segment, and preferably each segment, is configured to restrict the annular space between the plunger segment and the inner surface of the pump barrel. This restriction may be provided by ring seals or a close tolerance between the outer surface of the plunger segment and the pump barrel. In some embodiments, at least one segment defines at least one circumferential groove housing a seal (106) for sealingly and slidably engaging the pump barrel bore. Preferably, each plunger segment (102) defines a plurality of seal grooves each housing a fluid-energized seal (106). The seal need not be an O-ring or be a fluid-tight seal; small gaps when installed may be tolerated, so long as there is significant resistance to fluid flow around and past the plunger (100).

As a result of having a sufficient effective sealing elements, the seal between the plunger segment and the pump barrel provides sufficient resistance to fluid movement that it is suitable for this application. On the plunger downstroke, production fluid below the traveling valve should be forced into the production path within the plunger segments, and not leak out between the plunger and the pump barrel. The overall sealing ability produced by the multiple seal grooves and seals is designed to incorporate the equivalent seal area length as is well known in the art.

Other plunger finishes as are well known in the art may be utilized in this design. For example, spray metal, chrome plating, nitriding, or other advanced materials such as nickel based alloys, stainless steel, etc. may be employed. Any combination of plunger type and finish considered suitable by those skilled in the art may also be utilized.

As shown in FIG. 3, in some embodiments, the articulating connector (108) comprises a collar (110) and a plunger nipple (112) having a ball (114) portion and a shank (116). The collar is threaded onto the lower end of a plunger segment, while the plunger nipple is threaded into the upper end of an adjacent plunger segment. The connector is hollow to allow passage of production fluids, and sealed to contain those fluids.

In some embodiments, the collar (110) is fixed to the plunger segment with a straight close tolerance threaded connection, leaving a small gap between the upper end of the collar (110) and an external shoulder (120). An internal shoulder (122) on the socket, below the threaded portion, abuts a corresponding shoulder (124) on the plunger pin. Additionally, an inner bore surface (126) on the collar (110), above shoulder (122), is close tolerance fitted around an outer surface (128) on the plunger pin (130). In some embodiments, the combination of this close tolerance fitment and the internal abutment permits isolation of the large, static connection stresses from the large alternating stresses carried by the thicker portion of the collar wall below the internal abutment.

The pin (130) defines a partial hemispherical socket and the collar (110) also defines a partial hemispherical socket, which combine to retain the plunger nipple (112) ball, as is shown in the Figures. The diameter of the collar (110) socket is larger than the diameter of the pin (130) socket.

The plunger nipple (112) comprises a ball (114) and a shank (116) portion, which comprises external threads to engage a box threaded end of a plunger segment. An upper portion of the ball (114) has a diameter matching the diameter of the pin socket, while a lower portion of the ball has a larger diameter, matching the diameter of the collar (110) socket. A seal (132), such as an O-ring, may be provided between an upper portion of the ball and the pin socket to provide a seal and to prevent fine solids from infiltrating the socket. Alternatively, or additionally, an O-ring seal may be provided on a lower portion of the ball, in the region of the collar (110) socket. Alternatively, a polished metal-on-metal surface seal may be used. A secondary metal seal may be formed in the socket by an internal abutment metal-on-metal shoulder between the collar (110) and the plunger segment.

In some embodiments, a travel limiting shoulder (134) is formed around the surface of the ball, which limits rotation of the ball within the socket by abutting against the pin (130) end of the plunger segment. The lower end of the collar (110) has a flared opening, allowing for pivoting movement of the shank. In preferred embodiments, the ball shoulder travel stop (134) is configured such that shank does not contact the collar (110) at the travel limits of the ball within the socket, as is illustrated in FIG. 4. The shank itself may have a reduced diameter or relief and/or the flared opening may have some additional relief in the area of potential contact. This mitigates against unwanted contact between the two faces, which contact could introduce external forces and stresses.

The ball itself comprises a reduced diameter band (136), which may be machined flat, around the circumference of the ball, just below the ball shoulder (134). Preferably this band is located at about the equator of the ball. This band also modifies contact and stress transfer between the plunger nipple (112) and the collar (110), making the articulating connector more robust. Additionally, this reduced outside diameter is desired to permit passage of the plunger nipple (112) through the collar (110) internal shoulder during assembly.

The articulating plunger joint connection is assembled by first installing a seal, such as an o-ring, into the groove on the upper portion of the ball; which is then passed through the inside of the collar (110), leading with the male thread end until the lower hemispherical phase of the articulating joint connector meets with the internal socket of the collar (110). Finally, the collar (110) is threaded to the pin end of a plunger segment until the internal abutment features meet and the prescribed torque is applied. On the bottom end, the male threaded connector is rigidly connected to an adjacent plunger component.

In some embodiments, the same basic plunger body can be employed with one instance or multiple instances and in either condition being easily interchangeable with standard pump componentry. Therefore, in some embodiments, the articulating plunger is configured to utilize identical plunger bodies throughout the assembly. As shown in FIG. 5, the top of the upper most plunger segment (102) is fitted with an adapter (200) which then allows connection to a standard plunger top cage (202) and ultimately the valve rod (12). The plunger segments may have non-standard threads, in which case the adapter (200) may have standard API threads to allow connection to a standard plunger top cage.

In some embodiments, the invention comprises a top rod articulating joint, which permits rotation and back spin of the rod string above the pump while preventing rotation of the pump internals. The rod articulation joint must provide a robust and rotatable connection which does not mechanically release due to unthreading of the internal connection. Therefore, in some embodiments, a pump top articulating joint is configured to incorporate anti-rotation connection features to prevent the pre-mature release of the threaded connections during normal pumping operations.

In some embodiments, a top articulating joint (10) connects between the bottom of the rod string (1) and the top of the valve rod (12). The top articulating joint (10) allows for deflection between the rod string and the valve rod, and also swivels to allow free rotation of the rod string without rotating the valve rod. This functionality may reduce stresses due to bending on the upstroke of the rod string at the top face of the fixed valve rod guide (14), and help mitigate uneven wear of the rod string centralizers (not shown).

A collet-style rod bushing (302) is connected to the top of the valve rod (12) and secured by a rod bushing nut (304). The rod sub (306) is threaded into the rod string (1) which extends to the pumping unit on surface. A swivel sub (308) has a crowned upper surface (310) which bears on a lower surface (312) of the rod sub (306). The lower end of the swivel sub is threaded to the rod bushing (302) and has an exposed shoulder (314). A collar (316) rigidly threads onto the rod sub (306) and has a lower lip (318) which retains the swivel sub by bearing on the shoulder (314). The swivel sub may be equipped with a hex head to receive a socket wrench for ease of assembly.

There is sufficient clearance within the collar (316) to allow the swivel sub (308) to pivot slightly in relation to the rod sub (306), while still being retained by the collar lower lip (318). In addition, the swivel sub (308) may freely rotate within the collar (316).

In some embodiments, at least one spring pin (320) is installed axially through the swivel sub (308) and into the top of the rod bushing (302) male thread end after the components are torqued together. The spring pins mechanically lock together the swivel sub and the rod bushing and thereby prevent the components from unthreading in pumping operation.

The articulating rod joint is assembled by first passing the top pin end connection of the rod bushing (302) through the bottom of the collar (316) and the threading the swivel sub (308) onto the top end of the rod bushing (302) after passing it through the top end of the collar (316). The connection is then torqued together, subsequently two holes are drilled, which holes pass through the swivel sub length and into the pin end connection of the rod bushing. The spring pins (320) inserted into the holes and locked in place. Next, the sub-assembly comprised of rod bushing (302), collar (316) and swivel sub (308) is threaded and torqued onto the rod sub (306) which in turn connects to the rod string (1) to surface. On the bottom end, the rod bushing nut (304) is passed over the valve rod (12) which in turn is threaded into the rod bushing (302) internal thread. Finally, the rod bushing nut (304) is passed over the collet end and threaded and torqued onto the rod bushing (302) external thread which locks the rod bushing assembly onto the pump valve rod (12).

While the illustrated embodiment of the rod articulating joint comprises a collet style valve rod bushing connection, alternative embodiments of the rod articulating joint may be furnished with any style of valve rod bushing as required by the application or desired by the operator.

As will be apparent to those skilled in the art various modifications of this design may be employed without departing significantly from the aspects of this invention. For example, alternate materials may be selected for the base components provided that the mechanical properties of the same meet the minimum basic requirements expected of the components in application.

DEFINITIONS AND INTERPRETATION

The description of the present invention has been presented for purposes of illustration and description, but it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. To the extent that the following description is of a specific embodiment or a particular use of the invention, it is intended to be illustrative only, and not limiting of the claimed invention.

The corresponding structures, materials, acts, and equivalents of all means or steps plus functional elements in the claims appended to this specification are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed.

References in the specification to “one embodiment”, “an embodiment”, etc., indicate that the embodiment described may include a particular aspect, feature, structure, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to combine, affect or connect such aspect, feature, structure, or characteristic with other embodiments, whether or not such connection or combination is explicitly described. In other words, any element or feature may be combined with any other element or feature in different embodiments, unless there is an obvious or inherent incompatibility between the two, or it is specifically excluded.

It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as “solely,” “only,” and the like, in connection with the recitation of claim elements or use of a “negative” limitation. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

The singular forms “a,” “an,” and “the” include the plural reference unless the context clearly dictates otherwise. The term “and/or” means any one of the items, any combination of the items, or all of the items with which this term is associated.

As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. A recited range (e.g., weight percents or carbon groups) includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, all language such as “up to”, “at least”, “greater than”, “less than”, “more than”, “or more”, and the like, include the number recited and such terms refer to ranges that can be subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio.

The term “about” can refer to a variation of ±5%, ±10%, ±20%, or ±25% of the value specified. For example, “about 50” percent can in some embodiments carry a variation from 45 to 55 percent. For integer ranges, the term “about” can include one or two integers greater than and/or less than a recited integer at each end of the range. Unless indicated otherwise herein, the term “about” is intended to include values and ranges proximate to the recited range that are equivalent in terms of the functionality of the composition, or the embodiment. 

1. A reciprocating rod pump system, comprising an articulating plunger assembly comprising: (a) at least two plunger segments, each comprising a pin end and a box end, wherein the pin end has an external threaded portion and the box end has an internally threaded portion; (b) a ball and socket articulating connector, formed between the pin end of one plunger segment and the box end of another plunger segment, which articulating connector comprises: i. a collar rigidly attached to the pin end, the collar and pin end together defining a socket, the collar comprising an internal shoulder which abuts an external shoulder on the pin end, such that the uppermost end of the collar does not contact the plunger segment; ii. a plunger nipple comprising a ball and a shank, wherein the ball is adapted to fit and rotate within the socket, and the shank comprises an external thread which rigidly engages the box end thread of a plunger segment.
 2. The rod pump system of claim 1 wherein the ball comprises a travel limiting abutment which contacts a lower surface of the pin end at the rotational travel limit of the ball within the socket, and the shank and collar are configured so as to not contact each other at the travel limit.
 3. The rod pump system of claim 1 wherein an inner bore surface on the collar, above the internal shoulder, is close tolerance fitted around an outer surface on the pin end.
 4. The rod pump system of claim 3 wherein the ball comprises a travel limiting abutment which contacts a lower surface of the pin end at the rotational travel limit of the ball within the socket, and the shank and collar are configured so as to not contact each other at the travel limit.
 5. The rod pump system of claim 2 wherein the ball comprises a reduced diameter band adjacent the travel limiting abutment.
 6. The rod pump system of claim 5 wherein the reduced diameter band is located on or about the equator of the ball.
 7. A reciprocating rod pump system comprising a top rod articulating joint for connecting between a reciprocating rod string and a valve rod comprising: (a) a rod sub configured to connect to the rod string and having a lower bearing surface; (b) a swivel sub having an upper bearing surface and lower internal threaded portion and a collar which engages the rod sub to retain the swivel sub, which can freely rotate within the collar; (c) a rod bushing having a lower end adapted to connect to a valve rod and a threaded upper end which rigidly engages the swivel sub internal threaded portion; and (d) at least one pin, disposed longitudinally within the swivel sub and the upper end of the rod bushing.
 8. The rod pump system of claim 7 comprising two pins.
 9. An articulating plunger assembly for use in a reciprocating rod pump system, the plunger assembly comprising (a) at least two plunger segments, each comprising a pin end and a box end, wherein the pin end has an external threaded portion and the box end has an internally threaded portion; (b) a ball and socket articulating connector, formed between the pin end of one plunger segment and the box end of another plunger segment, which articulating connector comprises: i. a collar rigidly attached to the pin end, the collar and pin end together defining a socket, the collar comprising an internal shoulder which abuts an external shoulder on the pin end, such that the uppermost end of the collar does not contact the plunger segment; ii. a plunger nipple comprising a ball and a shank, wherein the ball is adapted to fit and rotate within the socket, and the shank comprises an external thread which rigidly engages the box end thread of a plunger segment.
 10. The plunger assembly of claim 9 wherein the ball comprises a travel limiting abutment which contacts a lower surface of the pin end at the rotational travel limit of the ball within the socket, and the shank and collar are configured so as to not contact each other at the travel limit.
 11. The plunger assembly of claim 9 wherein an inner bore surface on the collar, above the internal shoulder, is close tolerance fitted around an outer surface on the pin end.
 12. The plunger assembly of claim 10 wherein the ball comprises a reduced diameter band adjacent the travel limiting abutment.
 13. The plunger assembly of claim 12 wherein the reduced diameter band is located on or about the equator of the ball.
 14. The plunger assembly of claim 9 wherein all plunger segments are identical.
 15. (canceled)
 16. The rod pump system of claim 4 wherein the ball comprises a reduced diameter band adjacent the travel limiting abutment.
 17. The plunger assembly of claim 10 wherein an inner bore surface on the collar, above the internal shoulder, is close tolerance fitted around an outer surface on the pin end.
 18. The plunger assembly of claim 11 wherein the ball comprises a reduced diameter band adjacent the travel limiting abutment. 